01 Hans lived with his master in Berlin, Germany. One day Mr. von Osten invited some friends to his house. He led them to a courtyard where the horse was waiting quietly. "Are you ready, Hans?" he asked.
02 And the horse nodded!
03 "How much is four plus three?" Mr. von Osten asked. Hans raised his right foreleg and began to tap his hoof on the old stone floor of the courtyard. "One, two, three," he tapped, "four, five, six, seven"—and stopped.
04 Everyone began to talk at once. Mr. von Osten just smiled—and asked another question.
05 Mr. von Osten spread out six squares of cloth, each a different color. "Pick up the green one," he ordered. Hans walked over and stopped in front of the green square, picked it up in his teeth, and carried it back to his master.
06 For the next hour, Mr. von Osten asked questions—and Hans answered them. He was right almost every time.
07 All this happened many years ago, when there was no radio or television. Slowly word of the horse and what he could do spread through Berlin, then all of Germany—and at last into other countries. More and more people came to the von Osten courtyard to see the wonder horse perform.
08 Almost every day Hans showed his eager audiences some new talent. He could tell all sorts of things apart—even if they were almost the same size or shade or shape. Hans could also give the right answer when asked the time.
09 Hans had one talent that amazed people more than all the rest. Mr. von Osten could stand in front of the horse and just think of a question. He didn't move his lips or make the slightest sound. Yet Hans would answer the question anyway. Clever Hans could read his master's mind!
10 But not everyone agreed that Hans was a real thinking horse. Paul Bushe, a circus animal trainer, watched Mr. von Osten very carefully to see if he was sending signals to the horse to give him the right answer. After careful study, Mr. Bushe admitted that Hans was not getting signals from Mr. von Osten.
11 Still, people had questions. One of these was a scientist named Oskar Pfungst. Other people had studied Hans for a few hours or a few days. Professor Pfungst would work for as long as it took to finally solve the mystery of Clever Hans.
12 Professor Pfungst started out asking questions, just as other people had done. Hans answered easily. Then one day the scientist thought of something new. He asked the horse a question he didn't know the answer to himself. "How far is it from Berlin to London, England?" he asked.
13 Poor Hans tried again and again to answer that question. But he couldn't do it. The Professor grew more and more excited. He kept asking questions. When he asked a question he knew the answer to, Hans knew the answer. When he asked a question that he didn't know the answer to, Hans didn't either.
14 Before the day was over, Professor Pfungst knew that Hans couldn't really add or subtract or multiply or divide. He couldn't tell colors or coins or playing cards apart. He couldn't read or tell the time. Hans wasn't a thinking horse at all. He only "knew" as much as the person who was questioning him—and no more!
15 That meant that the person questioning Hans was signaling him. But how? Even the Professor himself must be sending signals—but he had no idea how he was doing it.
16 Day after day, Professor Pfungst asked Hans questions. He watched as many other people questioned the horse. Little by little, he began to understand.
17 Most trained animals can follow signals—like a hand movement or a change in the tone of voice. But none of these planned signals had ever been used with Hans. No, Professor Pfungst announced; people who questioned Hans were signaling Hans even though they did not mean to.
18 First the person asked Hans a question—and naturally he grew a little tense as he waited for the horse's answer. When this happened, many tiny body changes began to take place—changes the person wasn't trying to make at all. He might swallow a few more times than usual. His lips might tighten. Or one of his eyebrows would give the slightest twitch. These signs of tension told Hans to start giving his answer.
19 Suppose the person had asked Hans how much five plus five is. With each tap of Hans's hoof, the person got more and more tense. 1—2—3—4—5—6—7—8—9— Then, as Hans tapped 10, the person relaxed.
20 Now another whole group of tiny changes began to take place. The person might take a slightly deeper breath—or begin to breathe more slowly. His lips might open a little. His skin might even grow a bit pinker. All these tiny signs of relaxation told Hans to stop.
21 When someone wanted Hans to nod yes, he couldn't help making some kind of upward motion himself. And when someone wanted Hans to walk over to something he couldn't help making some small movement in that direction. Hans would wander around until he happened to pass in front of what the person was thinking about. Then the person would relax—and Hans would stop. He had given the "right" answer again.
22 So Clever Hans couldn't really think— not the way people do. Yet he was still a very special horse. He had puzzled one expert after another for a long, long while. He might not have been able to read minds—but he was one of the champion muscle readers of all time!
